Magneto-optical device

ABSTRACT

It is possible to reduce the size of a magneto-optical device, increase the speed of optical control, simplify power supply structure and its control, and maintain a Faraday rotation angle in an arbitrary state even after shut-off of the excitation current. The magneto-optical device includes a magnetic yoke ( 10 ) made of a high-permeability magnetic material, the magnetic yoke including a tabular portion ( 16 ) and four pillar portions ( 18 ) protruding from one side of the tabular portion ( 16 ), a coil ( 12 ) wound on each of the pillar portions, and a magneto-optical element ( 14 ) arranged in an open-magnetic-circuit region surrounded by the end portions of the four pillar portions. A magnetic field obtained by a coil is applied to the magneto-optical element.

TECHNICAL FIELD

The present invention relates to a magneto-optical device utilized in avariable optical attenuator, an optical switch, or the like, andparticularly to a magneto-optical device in which a magnetic yoke havingat least three pillar portions protruding from a tabular portion isutilized, and the magnetization direction of a magneto-optical elementis controlled by a magnetic field generated through coils wound aroundthe columnar portions.

BACKGROUND ART

In an optical communication system, an optical measurement system, andthe like, a variable optical attenuator is incorporated that is a devicefor variably controlling the transmitted optical power. The device iscomprised of a magneto-optical element having the Faraday effect, apermanent magnet that applies a fixed magnetic field to themagneto-optical element, and an electromagnet that applies a variablemagnetic field to the magneto-optical element. Typically, theelectromagnet is of a structure in which a coil is wound around aC-shaped (a shape of a circle having an open portion) magnetic yoke. Byinserting the magneto-optical element into the open portion of theC-shaped magnetic yoke and applying electric current to the coil, adesired variable magnetic field is applied to the magneto-opticalelement.

The direction of a constant magnetic field H_(r) generated by thepermanent magnet is made approximately parallel with the optical axis ofthe magneto-optical element, the magneto-optical element is magneticallysaturated, and the maximal Faraday rotation in an actual use is caused.In other words, the Faraday rotation angle θ_(f0) in the Faradayarrangement of the permanent magnet and the magneto-optical element andthe maximal Faraday rotation angle θ_(fMax) in an actual use are made tobe equal (θ_(f0)=θ_(fMax)). Next, a magnetic field H_(v) whose directionis approximately perpendicular to the direction of the magnetic fieldH_(r) generated by the permanent magnet is generated by theelectromagnet, the magneto-optical element is arranged in the combinedmagnetic field formed of the magnetic field of the permanent magnet andthe magnetic field of the electromagnet, and the magnitude of themagnetic field generated by the electromagnet is varied in accordancewith the magnitude of the current that flows in the coil of theelectromagnet, whereby the direction of the combined magnetic field iscontrolled. The polarization direction can be controlled in accordancewith the magnitude of the optical-axis-direction component of thecombined magnetic field. The direction θ_(c) of the combined magneticfield H_(c) is given by the following equation:θ_(c)=tan⁻¹(H _(v) /H _(r))Variation of the variable magnetic field H_(v) varies the directionθ_(c) of the combined magnetic field H_(c). The Faraday rotation angleθ_(f) is in accordance with the optical-axis-direction component of thecombined magnetic field H_(c), and thus given by the following equation;therefore, θ_(f) can be controlled by varying θ_(c).θ_(f)=θ_(f0)×cos θ_(c)In other words, by controlling the current that flows in the coil of theelectromagnet for generating H_(v), θ_(f) can be controlled.

The Faraday rotation θ_(f0) is caused through the fixed magnetic fieldH_(r) generated by the permanent magnet, and the Faraday rotation θ_(f)is obtained in accordance with the direction θ_(c) of the combinedmagnetic field H_(c) formed of the variable magnetic field H_(v)generated by the electromagnet and fixed magnetic field H_(r) generatedby the permanent magnet. Because, being a magnetic field generated by apermanent magnet, H_(r) is constant and not enabled to be zero, largeH_(v) is required to obtain large θ_(c). In order to obtain large H_(v),it is necessary to increase the number of windings of the electromagnetcoil or to increase current to be applied to the coil; therefore, thesize of the electromagnet is enlarged or the driving voltage isincreased. Moreover, there has been a problem in that it takes a longtime until the change of the direction θ_(c) of the combined magneticfield after the driving voltage has been changed, i.e., operating speedis low.

The case of a variable optical attenuator has a structure in which afirst light polarizer, a magneto-optical element, and a second lightpolarize are arranged in that order, along the optical axis, asaturation magnetic field is applied through a permanent magnet to themagneto-optical element, and a variable magnetic field whose directionis different from that of the saturation magnetic field is appliedthrough an electromagnet to the magneto-optical element. Through thepermanent magnet and the electromagnet, external magnetic fields areapplied in two or more directions to the magneto-optical element, andthe direction of magnetization of the magneto-optical element is changedby changing the vector of the combined magnetic field produced by thepermanent magnet and the electromagnet, whereby the Faraday rotationangle of light that passes through the magneto-optical element iscontrolled. For example, Japanese Patent Laid-Open No. 9-061770discloses a magneto-optical device, as described above, in which aconfiguration is employed where block-shaped permanent magnets arearranged above and below the light path, as a means for applying a fixedmagnetic field. Additionally, there is a configuration in whichpermanent magnets of ring-shape or the like are arranged along theoptical axis, and a fixed magnetic field is applied in parallel with theoptical axis.

As described above, since, in a conventional variable opticalattenuator, a permanent magnet is utilized to apply a saturationmagnetic field, the magneto-optical element is magnetized in a constantdirection by the permanent magnet, even when no electric current issupplied to the electromagnet. Accordingly, the size of a magnetic yokefor an electromagnet utilized for the control of magnetization isrendered large, or the driving voltage is made large; therefore, it isdifficult to downsize and speed up the variable optical attenuator.

Moreover, because, when no electric current is applied to the coil ofthe electromagnet, the variable magnetic field generated by theelectromagnet becomes substantially zero, the combined magnetic field tobe applied to the magneto-optical element consists only of the componentgenerated by the permanent magnet, whereby the Faraday rotation anglereturns to the initial condition.

In contrast, in a Faraday rotator utilized as a self-latching opticalswitch or the like, the Faraday rotation angle does not returns to theinitial condition, even after the exciting current for the coil has beencut off; therefore, the condition in the case where the electric currentis applied can be maintained. A Faraday rotator having the self-latchingfunction is comprised of a magneto-optical element having the Faradayeffect and a magnetic-field applying device that applies a magneticfield to the magneto-optical element; normally, no permanent magnet isutilized, and the magnetic-field applying device consists only of anelectromagnet. As the electromagnet, for example, as disclosed inJapanese Patent Laid-Open No. 8-211347, a structure is utilized in whicha coil is wound around a C-shaped magnetic yoke. By inserting themagneto-optical element into the open portion of the C-shaped magneticyoke and supplying electric current to the coil, a magnetic field isapplied to the magneto-optical element. Typically, the control isimplemented, with the absolute value of the electric current keptconstant, through polarity-reverse action of the electric current,accordingly, the number of possible directions for a magnetic fieldapplied to the magneto-optical element is only two, i.e., the positiveand negative directions along a single line; therefore, the number ofpossible states for the Faraday rotation angle is limited to two states.

In addition, in the Faraday rotator, at least one of the magnetic yokeand the magneto-optical element is formed of a semi-hard magneticmaterial; both of them are magnetized by exciting current; and after theexciting current is cut off, magnetization remains in the semi-hardmagnetic material. In the case where the magneto-optical element isformed of a semi-hard magnetic material, magnetization remains in themagneto-optical element itself; however, in the case where the magneticyoke is formed of a semi-hard magnetic material, a magnetic fieldgenerated by residual magnetization of the magnetic yoke is applied tothe magneto-optical element. In both cases, the residual magnetic-fieldvector and the magnetic-field vector in the case where the electriccurrent is applied are different in magnitude, but the same indirection. Accordingly, even after the exciting current has been cutoff, the Faraday rotation angle can be maintained in the same conditionas that in the case where the exciting current is flowing; however,maintainable are only two conditions that are possible when the electriccurrent is applied, whereby arbitrary condition cannot be maintained.

As described above, in a conventional variable optical attenuator, bycontrolling the exciting current supplied to the coil of theelectromagnet, the magnetization direction of the magneto-opticalelement is arbitrarily changed, and, in response to the change of themagnetization direction, the Faraday rotation angle can arbitrarilyadjusted; however, the Faraday rotation angle cannot be maintained afterthe exciting current has been cut off. In contrast, in a conventionalFaraday rotator having a self-latching function, even after the excitingcurrent has been cut off, the magnetization direction of themagneto-optical element and the Faraday rotation angle can bemaintained; however, the number of maintainable conditions is limited totwo.

DISCLOSURE OF THE INVENTION

The first issue to be solved by the present invention is that, in thecase of a conventional magneto-optical device in which a permanentmagnet and a electromagnet are combined, because a magnetic fieldgenerated by the permanent magnet always acts, it is necessary toincrease the number of windings of a coil that forms a variable magneticfield or to increase an electric current to be supplied, wherebydownsizing is impossible, and it is difficult to speed up the control oflight. The second issue to be solved by the present invention is that,even though the permanent magnet is simply replaced by theelectromagnet, the complexity of the configuration of the power supplyunits is raised, and the control is rendered difficult. The third issueto be solved by the present invention is that, in the case of aconventional magneto-optical device in which a permanent magnet and aelectromagnet are combined, the Faraday rotation angle cannot bemaintained after the exciting current has been cut off, and in the caseof a conventional magneto-optical device having a self-latchingfunction, the number of conditions in which the magnetization directionof the magneto-optical element and the Faraday rotation angle can bemaintained after the exciting current has been cut off is not arbitrarybut limited to two.

[The First Aspect of the Present Invention]

According to the first aspect of the present invention, there isprovided a magneto-optical device comprising: a magnetic yoke made of ahigh-magnetic-permeability material, the magnetic yoke including atabular portion, and at least three pillar portions protrudingperpendicularly from one side of the tabular portion; coils wound aroundthe pillar portions; and a magneto-optical element arranged in anopen-magnetic-circuit space surrounded by the respective top-endportions of the pillar portions, wherein magnetic fields generatedthrough the coils are applied to the magneto-optical element.

The simplest magnetic yoke has a structure in which an approximatelyquadrangular tabular portion and four quadrangular pillar portionsprotruding from the vicinities of the four corners of the tabularportion, perpendicularly and in the same direction, are continuouslyintegrated. In this case, it is preferable to employ a square tabularportion as well as a square pillar portion.

For the foregoing members, it is preferable that, as thehigh-magnetic-permeability material, ferrite (e.g., Ni—Zn systemferrite) is utilized, and for the magneto-optical element, abismuth-substituted rare-earth iron-garnet single crystal is utilized.Besides, by controlling respective directions and/or values of electriccurrents supplied to the coils, the magnetization direction of themagneto-optical element can be changed.

By arranging light polarizers before and after the foregoingmagneto-optical device in the light path thereof and controlling anattenuation value of output optical power versus input optical power, avariable optical attenuator can be configured. Additionally, byarranging light polarizers before and after the magneto-optical devicein the light path thereof and controlling switchedly output light versusinput light, an optical switch can be configured.

By arranging side by side a plurality of the foregoing magneto-opticaldevices, various kinds of magneto-optical device arrays can beconfigured.

The magneto-optical device according to the first aspect of the presentinvention can be utilized in a variable optical attenuator or an opticalswitch. When the magneto-optical device is utilized in a variableoptical attenuator, fixed magnetic field of a permanent magnet is notnecessary, and by generating variable magnetic fields by utilizingelectromagnets only, the magnetization direction can instantaneously bechanged, whereby downsizing and speedup can be implemented. In addition,since the magnetization direction can arbitrarily be controlled, therange from −45° to +45°, instead of the range from 0° to 90°, can beutilized so that the required 90-degree variable amount in the Faradayrotation angle is obtained; therefore, also in that sense, thedownsizing of components to be utilized can be achieved. When themagneto-optical device is utilized in an optical switch, by utilizing amagneto-optical element having a self-latching function, it is notrequired to utilize a semi-hard magnetic material as a magnetic-yokematerial; therefore, the magnetic field generated through the coils andthe magnetic yoke may be a critical mass for reversing the magnetizationof the magneto-optical element, whereby it is possible to achievedownsizing, speedup, and reduction of power dissipation. Moreover,because the magnetic-optical device has a structure in which a leakagemagnetic field is weak and magnetic fields converge only on themagneto-optical element, they do not interfere with one another eventhough a plurality of the magneto-optical devices are arranged side byside; therefore, an array configuration can readily be realized.

[The Second Aspect of the Present Invention]

According to the second aspect of the present invention, there isprovided a magneto-optical device comprising: a magnetic yoke made of ahigh-magnetic-permeability material, the magnetic yoke including 2n(where n≧2) pillar portions protruding perpendicularly from one side ofa tabular portion; coils wound around the pillar portions; and amagneto-optical element arranged in an open-magnetic-circuit spacesurrounded by the respective top-end portions of the pillar portions,wherein: magnetic fields generated through the coils are applied to themagneto-optical element; the polarities of magnetic fields applied tothe magneto-optical element, through the coils diagonally opposing eachother with respect to the magneto-optical element, are reverse to eachother; and a pair of the coils that are diagonally opposing each otherwith respect to the magneto-optical element and connected in parallel orin series is driven by a common power supply unit. In addition, thepower supply unit may be a variable voltage source or a variable currentsource.

The most simple magnetic yoke has a structure having an approximatelysquare tabular portion and four pillar portions protrudingperpendicularly and in the same direction, from the vicinities of thefour corners of the tabular portion. In this situation, the tabularportion and the pillar portions maybe in an integrated structure, or astructure may be employed in which each pillar portion is inserted intoholes provided in the tabular portion and fixed therein.

A structure may be employed in which a magnetic yoke has a tabularportion and pillar portions protruding in the same direction from theone side of the tabular portion, one pair of the pillar portionsopposing each other with respect to the magneto-optical element isarranged perpendicular to the optical axis of the magneto-opticalelement and the other pair of the pillar portions opposing each otherwith respect to the magneto-optical element is arranged at a specificangle (smaller than ±90°) from the optical axis. In the case of theforegoing structure, it is preferable that the angle θ_(h) between thedirection of a first magnetic field formed through the one pair ofpillar portions and the direction of a second magnetic field formedthrough the other pair of pillar portions is set to the angle given bythe following equation:θ_(h)=sin⁻¹(θ_(fMAX)/θ_(f0))where θ_(f0) is the Faraday rotation angle of the magneto-opticalelement in the case where the direction of a saturation combinedmagnetic field is parallel to the light path, and θ_(fMAX) is a maximalFaraday rotation angle of the magneto-optical element in the case ofactual use.

Further, according to the second aspect of the present invention, thereis provided a magneto-optical device in which two magnetic yokes made ofa high-magnetic-permeability material and having 2n (where n≧2) pillarportions protruding perpendicularly from one side of a tabular portionare combined in such a way that, with respective coils wound around thepillar portions, the foremost surfaces of the pillar portions of onemagnetic yoke butt against the foremost surfaces of the correspondingpillar portions of the other magnetic yoke, a magneto-optical element isarranged in an open-magnetic-circuit space surrounded by the respectivetop-end portions of the pillar portions, and magnetic fields generatedthrough the coils being applied to the magneto-optical element, wherein:the coils wound around the corresponding pillar portions generatemagnetic fields that are reverse to each other; the polarities ofmagnetic fields applied to the magneto-optical element, through thecoils opposing each other with respect to the magneto-optical element,are reverse to each other; the magnetic field generated through the onemagnetic yoke and the magnetic field generated through the othermagnetic yoke cooperate to act on the magneto-optical element; and apair of the coils diagonally opposing each other with respect to themagneto-optical element configure a set and are driven by a common powersupply unit.

Moreover, according to the second aspect of the present invention, thereis provided a magneto-optical device comprising: a magnetic yoke made ofa high-magnetic-permeability material, the magnetic yoke includingtabular portions opposing and spaced apart from each other and 2n (wheren≧2) pillar portions arranged between the tabular portions; a pluralityof coils wound around each of the pillar portions; and a magneto-opticalelement arranged in an open-magnetic-circuit space surrounded by therespective middle portions of the pillar portions, wherein: magneticfields generated through the coils are applied to the magneto-opticalelement; the coils wound around the same pillar portion generatemagnetic fields that are reverse to each other; the polarities ofmagnetic fields applied to the magneto-optical element, through thecoils opposing each other with respect to the magneto-optical element,are reverse to each other; assuming that the middle portion of thepillar portion regarded as a boundary, the magnetic field generatedthrough the one-side portion of the magnetic yoke and the magnetic fieldgenerated through the other-side portion magnetic yoke cooperate to acton the magneto-optical element; and a pair of the coils diagonallyopposing each other with respect to the magneto-optical elementconfigure a set and are driven by a common power supply unit.

With the foregoing configuration, it is preferable that, in the casewhere the direction of a saturation combined magnetic field is parallelto the optical axis of the magneto-optical element, the Faraday rotationangle of the magneto-optical element is set to 127.3° or larger, and thecoils are driven by monopolar power supply units.

As is the case with the magneto-optical device according to the firstaspect of the present invention described above, the magneto-opticaldevice according to the second aspect of the present invention can beutilized as a Faraday rotator in a variable optical attenuator or anoptical switch. Further, by generating magnetic fields by utilizingelectromagnets only, without utilizing any fixed magnetic field of apermanent magnet, the magnetization direction can instantaneously bechanged, whereby downsizing and speedup can be implemented. Since,regardless of the direction of the combined magnetic field, no largemagnetic field is required, the coils can be downsized, whereby thedriving voltage can also be reduced.

Since a pair of coils diagonally opposing each other with respect to amagneto-optical element is configured and the coils in the pair arewired in parallel or in series and driven by a common power supply unit,the drive of the coils can efficiently be implemented with a smallnumber of power supply units, whereby the peripheral circuitry can besimplified. In particular, if coils that are connected in parallel aredriven, the voltage can be reduced. Moreover, if, in the case where thedirection of a saturation combined magnetic field is parallel to theoptical axis of the magneto-optical element, the Faraday rotation angleof the magneto-optical element is set to 127.3° or larger, the Faradayrotation angle can be varied over the range from 0° to 90° even thoughthe coils are driven by monopolar power supply units.

By employing the magneto-optical device having a configuration in whichtwo magnetic yokes made of a high-magnetic-permeability material andhaving 2n (where n≧2) pillar portions protruding perpendicularly fromone side of a tabular portion, are combined in such a way that, theforemost surfaces of the pillar portions of one magnetic yoke buttagainst the foremost surfaces of the corresponding pillar portions ofthe other magnetic yoke, or a configuration in which a magnetic yokemade of a high-magnetic-permeability material and having tabularportions that are spaced apart from and opposing each other and 2npillar portions arranged between the tabular portions is utilized, alarger magnetic field can be applied to the magneto-optical element.

[The Third Aspect of the Present Invention]

According to the third aspect of the present invention, there isprovided a magneto-optical device comprising: a magnetic yoke having atabular portion made of a semi-hard magnetic material and at least threepillar portions, protruding from one side of the tabular portion, thatare made of a high-magnetic-permeability material; coils wound aroundthe pillar portions; and a magneto-optical element arranged in anopen-magnetic-circuit space surrounded by the respective top-endportions of the pillar portions, wherein magnetic fields generatedthrough the coils are applied to the magneto-optical element.

In the magneto-optical device configured as described above, whenrespective currents are supplied to the coils, a magnetic field isgenerated not only in the open-magnetic-circuit space but also in themagnetic yoke, whereby both the tabular portion and the pillar portionsthat configure the magnetic yoke are magnetized. When exciting currentsare cut off, the magnetization in the pillar portions made of asoft-magnetic material tends to transit in a direction along thehysteresis curve so as to lose its own magnetization; in contrast, thetabular portion made of a semi-hard magnetic material exhibit a residualmagnetization that magnetizes the pillar portions, whereby a combinedmagnetic field is formed in the open-magnetic-circuit space. Thecombined magnetic field is different in magnitude from that in the casewhere the currents are applied, but the same in direction. Accordingly,by controlling the ratios of the magnetomotive force generated throughthe coils, thereby controlling the condition of magnetization to bemaintained in the tabular portion, the direction of the combinedmagnetic field that remains in the open-magnetic-circuit space, afterthe exciting currents are cut off, can be controlled so as to be in anarbitrary direction.

The magnetic yoke may have a structure in which a tabular portion madeof a high-magnetic-permeability material and at least three pillarportions, protruding from one side of the tabular portion, that are madeof a semi-hard magnetic material, are incorporated. In the case of thisconfiguration, after the exciting currents are cut off, magnetizationremains in the pillar portions, and the residual magnetization forms acombined magnetic field in the open-magnetic-circuit space.

By arranging a light polarizer and a light analyzer before and after theforegoing magneto-optical device in the light path thereof andcontrolling an attenuation value of output optical power versus inputoptical power, a self-latching variable optical attenuator can beconfigured. Moreover, by arranging the magneto-optical device, a lightpolarizer, a light analyzer, and a wavelength plate in a predeterminedorder and controlling optical separation ratio through themagneto-optical device, a self-latching variable optical splitter can beconfigured.

Further, according to the third aspect of the present invention, thereis provided a magneto-optical device comprising: a magnetic yoke havinga block-shaped base portion made of a semi-hard magnetic material and atleast four pillar portions, made of a high-magnetic-permeabilitymaterial, extending from the base portion to the vicinity of a space tobe an open-magnetic-circuit space; coils wound around the pillarportions; and a magneto-optical element arranged in theopen-magnetic-circuit space surrounded by the respective top-endportions of the pillar portions, wherein magnetic fields generatedthrough the coils are applied to the magneto-optical element, in threeor more directions. In the case of this configuration, the direction ofthe combined magnetic field applied to the magneto-optical element canbe controlled so as to be in an arbitrary direction in a 3-dimentionalspace, and the magnetic direction can be maintained after the excitingcurrents are cut off.

As is the case with the magneto-optical devices according to the firstand second aspects described above, the magneto-optical device accordingto the third aspect of the present invention can be utilized as aFaraday rotator in a variable optical attenuator, a variable opticalsplitter, or an optical switch. Further, fixed magnetic field of apermanent magnet is not required, therefore, by generating variablemagnetic fields by utilizing electromagnets only, downsizing of themagneto-optical device can be implemented. Because, regardless of thedirection of the combined magnetic field, no large magnetic field isrequired, the coils can be downsized, whereby the driving voltage canalso be reduced.

Moreover, by controlling the directions and the values of the respectivecurrents applied to the coils, the direction (the magnetizationdirection of the magneto-optical element) of the combined magnetic fieldformed in the open-magnetic-circuit space can be controlled to be in anarbitrary direction, whereby the Faraday rotation angle can arbitrarilybe adjusted. Furthermore, by forming with a semi-hard magnetic materialthe tabular portion or the pillar portion of the magnetic yoke, evenafter the exciting currents are cut off, the direction of the combinedmagnetic field that remains in the open-magnetic-circuit space can becontrolled so as to be in an arbitrary direction, whereby the Faradayrotation angle can be maintained so as to be in an arbitrary condition.

In the case where the magneto-optical device is utilized as a Faradayrotator in a variable optical attenuator, the value of opticalattenuation can be maintained to be in an arbitrary condition even afterthe exciting currents are cut off. Still moreover, in the case where themagneto-optical device is utilized as a Faraday rotator in a variableoptical splitter, the separation ratio for input light can be maintainedto be in an arbitrary condition after the exciting currents are cut off.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an exploded perspective view illustrating Embodiment 1 of amagneto-optical device according to the present invention;

FIG. 1B is a side view of the magneto-optical device in FIG. 1A;

FIG. 1C is a plan view of the magneto-optical device in FIG. 1A;

FIG. 2 is a view for explaining the operation of the magneto-opticaldevice in FIG. 1A;

FIG. 3 is a graph representing a relationship between the Faradayrotation angle and the current, with the magnetization directionutilized as a parameter;

FIG. 4 is a view illustrating a configuration of a variable opticalattenuator or an optical switch;

FIG. 5 is an explanatory graph representing an example of the responseperformance of a magneto-optical device;

FIG. 6A is an explanatory view illustrating Embodiment 2 of amagneto-optical device according to the present invention;

FIG. 6B is an explanatory view illustrating a condition in whichair-core coils are mounted on the magnetic yoke in FIG. 6A;

FIG. 6C is an explanatory view illustrating a condition in which amagneto-optical element is arranged in the open-magnetic-circuit spacein FIG. 6B;

FIG. 7A is a diagram for explaining arrangement of coils;

FIG. 7B is a chart for explaining magnetic fields to be applied;

FIG. 8 is a set of diagrams illustrating the relationship between agroup of magnetic poles and the direction of a magnetic field to beapplied;

FIG. 9A is a circuit diagram illustrating wiring of coils and powersupply units, in the case where two coils each are connected inparallel;

FIG. 9B is a circuit diagram illustrating wiring of coils and powersupply units, in the case where two coils each are connected in series;

FIG. 10A is a graph representing a relationship between the Faradayrotation angle and the current, in the case where a magneto-opticalelement having θ_(f0) of 90° is utilized;

FIG. 10B is a graph representing a relationship between the Faradayrotation angle and the current, in the case where a magneto-opticalelement having θ_(f0) of 127.3° is utilized;

FIG. 11A is an explanatory view illustrating an example of anotherstructure of a magnetic yoke;

FIG. 11B is an explanatory view illustrating a condition in whichair-core coils are mounted on the magnetic yoke in FIG. 11A;

FIG. 11C is an explanatory view illustrating a condition in which amagneto-optical element is arranged in the open-magnetic-circuit spacein FIG. 11B;

FIG. 12 is a chart representing another example of arrangement of coilsand magnetic fields to be applied;

FIG. 13 is a graph representing another example of the relationshipbetween the Faraday rotation angle and the current;

FIG. 14 is an explanatory view illustrating an example of anotherstructure of a magneto-optical device;

FIG. 15A is an explanatory view illustrating an example of furtheranother structure of a magneto-optical device;

FIG. 15B is an explanatory view illustrating a condition in which themagnetic yokes in FIG. 15A are bonded;

FIG. 15C is an explanatory view illustrating a condition in which amagneto-optical element is arranged in the open-magnetic-circuit spacein FIG. 15B;

FIG. 16A is an explanatory diagram illustrating arrangement of coils;

FIG. 16B is a circuit diagram illustrating wiring of coils and powersupply units, in the case where two coils each are connected inparallel;

FIG. 16C is a circuit diagram illustrating wiring of coils and powersupply units, in the case where two coils each are connected in series;

FIG. 17A is an explanatory view illustrating an example of anotherstructure of a magneto-optical device;

FIG. 17B is a perspective view illustrating a condition after themagneto-optical device in FIG. 17A has been assembled;

FIG. 18 is an explanatory diagram illustrating an application example ofa magneto-optical device applied to a variable optical attenuator;

FIG. 19A is an explanatory graph representing an example of the opticalattenuation properties of a variable optical attenuator;

FIG. 19B is an explanatory graph representing an example of the responseproperties of a variable optical attenuator;

FIG. 20 is an explanatory view illustrating Embodiment 3 of amagneto-optical device according to the present invention;

FIG. 21A is an explanatory view illustrating an example of themagnetization direction of a magnetic yoke and the direction of acombined magnetic field, in the case where currents are applied;

FIG. 21B is an explanatory view illustrating an example of themagnetization direction of a magnetic yoke and the direction of acombined magnetic field, in the case where currents are cut off;

FIG. 22 is an explanatory view illustrating an example of anotherstructure of a magnetic yoke;

FIG. 23 is an explanatory view illustrating an example of furtheranother structure of a magnetic yoke;

FIG. 24 is an explanatory chart representing an example of the anglebetween the direction of a combined magnetic field and the travelingdirection of a ray;

FIG. 25A is a graph representing an example of the relationship betweenthe direction of a combined magnetic field and the current;

FIG. 25B is a graph representing an example of measurement on theFaraday rotation angle, in the case where currents are applied and afterthe currents are cut off;

FIG. 26 is an explanatory diagram illustrating an example of amagneto-optical device applied to a variable optical attenuator;

FIG. 27 is an explanatory graph representing an example of the opticalattenuation properties of the variable optical attenuator in FIG. 26;and

FIG. 28 is an explanatory diagram illustrating an example of amagneto-optical device applied to a variable optical splitter.

Description of Symbols 10 MAGNETIC YOKE 12 COIL 14 MAGNETO-OPTICALELEMENT 16 TABULAR PORTION 18 PILLAR PORTION 110 TABULAR PORTION 112PILLAR PORTION 114 MAGNETIC YOKE 116 COIL 120 MAGNETO-OPTICAL CRYSTAL122, 123, 126, and 127 POWER SUPPLY UNIT 124 and 128 CONTROL DEVICE 210MAGNETIC YOKE 211 TABULAR PORTION 212 PILLAR PORTION 215 COIL 220MAGNETO-OPTICAL ELEMENT

MODE FOR CARRYING OUT THE INVENTION EMBODIMENT 1

Embodiment 1 of a magneto-optical device according to the presentinvention will be explained with reference to FIGS. 1A to 5. In the caseof the most simple configuration of a magneto-optical device inEmbodiment 1, a single-piece magnetic yoke made of ahigh-magnetic-permeability material is utilized in which quadrangularpillar portions are protruding perpendicularly and identically oriented,from the vicinities of the four corners of a square tabular portion,respective coils are wound around the quadrangular pillar portions, anda magneto-optical element is arranged in an open-magnetic-circuit spacesurrounded by the top-end portions of the four quadrangular pillarportions. Besides, variable magnetic fields through the coils areapplied to the magneto-optical element.

FIGS. 1A to 1C illustrate an example of a magneto-optical deviceaccording to the present invention. FIG. 1A is an exploded perspectiveview; FIGS. 1B and 1C are a side view and a plan view, respectively. Themagneto-optical device includes a magnetic yoke 10, coils 12 woundaround the magnetic yoke 10, and a magneto-optical element 14. Themagnetic yoke 10 has a structure in which a square tabular portion 16and four square pillar portions 18 that are equal in length andprotruding perpendicularly and identically oriented, at the four cornersof the square tabular portion 16, are continuously integrated, and madeof a high-magnetic-permeability material. The coils 12 are wound aroundthe respective pillar portions 18. The magneto-optical element 14 isarranged in an open-magnetic-circuit space surrounded by the top-endportions of the four pillar portions 18. Accordingly, a magnetic fieldgenerated by the coils 12 is applied to the magneto-optical element 14.

As the high-magnetic-permeability material, for example, Ni—Zn systemferrite is utilized. The magnetic yoke 10 has a structure formed byintegrally molding the high-magnetic-permeability material in apredetermined form and sintering the molded material. The performance ofthe magnetic yoke 10 is enhanced by utilizing ferrite single crystal. Inthis case, the magnetic yoke 10 is produced by cutting out a portion offerrite in a predetermined shape from a block ferrite. As themagneto-optical element, for example, a bismuth-substituted rare-earthiron-garnet single crystal is utilized. The single crystal can be grownthrough the LPE (liquid-phase epitaxy) method. Additionally, a YIG(yttrium iron garnet) single crystal may be utilized.

In the case of the magneto-optical device, by controlling the directionsand/or values of electric currents supplied to four coils, themagnetization direction of the magneto-optical element can be changed.

FIG. 2 is a set of plan views for explaining the operation of amagneto-optical device according to the present invention. Therespective top ends of the pillar portions of the magnetic yoke areindicated by reference characters a, b, c, and d; the respective coilsthat correspond to the top ends and are wound around the pillar portionsare indicated by similar reference characters (coils 12 a to 12 d). Inthis situation, it assumed that, as indicated by an arrow, light passesthrough the magneto-optical element, from the left-hand side of FIG. 2to the right-hand side. The coils 12 a and 12 b are wound in series.Besides, a and b have opposite magnetic polarities. The coils 12 c and12 d are also wound in series. Further, c and d have opposite magneticpolarities. Magnetization directions are indicated by white arrows.

(In-Plane Magnetization)

Single-direction currents flow through the coils 12 a and 12 b andthrough the coils 12 c and 12 d. Accordingly, a and b are magnetized toN and S poles, respectively; similarly, c and d are magnetized to N andS poles, respectively. In consequence, a magnetic field perpendicular tothe optical axis is applied to the magneto-optical element 14, i.e.,in-plane magnetization (magnetization whose direction is parallel to theincident/exit plane of the magneto-optical element) is generated.

(45-Degree Magnetization)

A single-direction current flows through the coils 12 a and 12 b only.No current is applied to 12 c and 12 d. Accordingly, a and b aremagnetized to N and S poles, respectively; inconsequence, a magneticfield whose direction has a gradient of 45° with respect to the opticalaxis is applied to the magneto-optical element 14, i.e., 45-degreemagnetization (magnetization whose direction has a gradient of 45° withrespect to the incident/exit plane of the magneto-optical element) isgenerated.

(Perpendicular Magnetization)

A single-direction current flows through the coils 12 a and 12 b, and asingle-direction current whose direction is reverse to that in the caseof the in-plane magnetization flows through the coils 12 c and 12 d.Accordingly, a and b are magnetized to N and S poles, respectively; cand d are magnetized to S and N poles, respectively. In consequence, amagnetic field whose direction is parallel to the optical axis isapplied to the magneto-optical element 14, i.e., perpendicularmagnetization (magnetization whose direction is perpendicular to theincident/exit plane of the magneto-optical element) is generated.

At any rate, an electric current whose direction is always the sameflows through the coils 12 a and 12 b, whereby the top ends a and b aremagnetized always in the same polarity (e.g., a is magnetized always toN, and b, to S pole). From the state of in-plane magnetization to thestate of the 45-degree magnetization, a single-direction current flowsalso through the coils 12 c and 12 d; by reducing the value of thecurrent, arbitrary magnetization directions can be realized. From thestate of 45-degree magnetization to the state of the perpendicularmagnetization, a reverse-direction current flows through the coils 12 cand 12 d; by increasing the value of the current, arbitrarymagnetization directions can be realized. As described above, bycontrolling the values and directions of the currents applied to thecoils, the direction of the magnetic field can be controlled.

FIG. 3 is a graph representing an example of measurement results withregard to the relationship between the Faraday rotation angle and thecoil-current value, with the magnetization direction utilized as aparameter. Although depending on the structure and the material of themagnetic yoke, it can be seen that, when the value of the coil currentis larger than a specific value, the Faraday rotation angle is saturatedto a constant value.

FIG. 4 is an explanatory view illustrating an example of a variableoptical attenuator according to the present invention. A configurationis employed in which, before and after the magneto-optical device 20, afirst light polarizer 22 and a second light polarizer 24 are arrangedalong the light path. The magneto-optical device 20 may be identical tothat illustrated in FIGS. 1A to 1C; for simplicity, correspondingmembers are designated by the same reference characters.

In this situation, the magneto-optical element 14 is made thick enoughto cause a Faraday rotation angle of 45° or larger. In addition, thefirst light polarizer 22 and the second light polarizer 24 are linearpolarizers, such as an absorption-type polarizer, polarization glass(brand names “Polarcor”, “CUPO”, and the like), or a multilayer-typepolarizer, and arranged in such a way that their optical axes are withinplanes that are parallel to each other and have a predetermined angledifference (e.g., a predetermined angle difference of 45° or largerbetween the optical axes, when viewed with respect to the light path)between them.

By controlling through the magneto-optical device 20 the Faradayrotation angle of the magneto-optical element 14 over the range from−45° to +45°, the amount of light output from the second light polarizer24 versus the amount of input light to the first light polarizer 22 isvariably controlled.

Because, without utilizing any fixed magnetic field generated by apermanent magnet, the magnetic field for a variable optical attenuatorconfigured as described above is made up merely of variable magneticfields generated by electromagnets, the magnetization direction caninstantaneously be changed, whereby the operation of the variableoptical attenuator is speeded up. In addition, because the magnetizationdirection can arbitrarily be controlled, the range from −45° to +45° canbe utilized so that the 90-degree variable amount in the Faradayrotation angle, which is required of an optical attenuator, is obtained;therefore, in comparison to a conventional configuration, themagneto-optical element is required to have merely half of the rotationangle (i.e., a thickness corresponding to 45-degree rotation angle).

The conditions obtained are listed below under which, by utilizing amagnetic yoke that is made of Ni—Zn ferrite having a magneticpermeability of 2000 and 2.8 mm by 2.8 mm by 4.5 mm (length by width byheight) in size, a magnetic-field strength of 12000 A/m (150 Oe), whichis required to magnetically saturate the magneto-optical element orreverse the magnetization direction, is obtained:

-   Number of windings: 130-   Coil resistance (diameter of conductor φ=50 μm): 5.17Ω-   Magnetomotive force: 11.9 AT-   Current value: 96 mA-   Power consumption: 48 mW-   Time constant: 50.1 μsec

FIG. 5 is a graph representing an example of measurement result on theresponse speed of the magnetic circuit. The duration between the instantwhen the input voltages to be applied to both sets of coils wereconcurrently switched and the instant when the light output becamestable was measured. It can be seen in FIG. 5 that the response speedwas approximately 40 μsec. For reference's sake, with a conventionaloptical attenuator, the response speed was approximately 200 μsec. Theoptical attenuator according to the present invention does not require apermanent magnet; therefore, the response speed is raised.

As is the case with FIG. 4, an optical switch is configured in such away that, before and after a magneto-optical device, respective lightpolarizers are arranged along the light path of the magneto-opticaldevice. In the case of an optical switch, it is desirable to utilize amagneto-optical element having a self-latching function. That isbecause, thanks to the self-latching function, the optical switch canmaintain the state even though the coil current is cut off. As amagneto-optical element having a self-latching function, for example, abismuth-substituted rare-earth iron garnet single crystal havingresidual magnetization can be utilized; by, without applying heattreatment, directly utilizing a film grown through the LPE method, theself-latching function is demonstrated. By switching Faraday rotationangles through the magneto-optical device, output light versus inputlight can be switchably controlled. Accordingly, it is not necessarythat, as a conventional magneto-optical device, the magnetic-yokematerial is limited to a semi-hard magnetic material. Therefore, themagnetic field generated through the coils and the magnetic yoke may bea critical mass for reversing the magnetization of the magneto-opticalelement; as a result, it is possible to downsize the device and toreduce the power dissipation.

Moreover, the magneto-optical device according to the present inventionhas a structure in which no permanent magnet is utilized, the leakagemagnetic field is small, and the magnetic fields converge on themagneto-optical element only; therefore, even though a plurality ofmagneto-optical devices are provided in parallel, no magneticinterference occurs, whereby an array structure is readily enabled.Variable optical attenuators, optical switch arrays, and the like can berealized, by utilizing the magneto-optical device.

In addition, although, in Embodiment 1, a configuration is employed inwhich four pillar portions are provided on the tabular portion, thepresent invention is not limited to the configuration; e.g., aconfiguration may also be employed in which three, or five or morepillar portions are provided. If at least three pillar portions areprovided on the tabular portion, by controlling respective directionsand values of the currents supplied to the coils wound around the pillarportions, the combined magnetic field can arbitrarily be oriented, as isthe case with four pillar portions. Moreover, the shape of the tabularportion is not limited to a square; e.g., other polygons, a circle, orthe like may be employed.

EMBODIMENT 2

Embodiment 2 of a magneto-optical device according to the presentinvention will be explained with reference to FIGS. 6A to 19B. In thecase of the most simple configuration of a magneto-optical device inEmbodiment 2, as illustrated in FIGS. 6A to 6C, a single-piece magneticyoke 114 made of a high-magnetic-permeability material is utilized inwhich quadrangular pillar portions 112 are protruding perpendicularlyand in the same direction, from the vicinities of the four corners of asquare tabular portion 110 (refer to FIG. 6A). Respective coils 116 arewound around the quadrangular pillar portions 112 (refer to FIG. 6B); amagneto-optical element 120 mounted on a non-magnetic holding member 118is arranged in an open-magnetic-circuit space surrounded by the top endsof the four quadrangular pillar portions 112 (refer to FIG. 6C).Besides, the magneto-optical device is configured in such a way thatvariable magnetic fields generated through the coils 116 are applied tothe magneto-optical element 120.

As illustrated in FIG. 7A, the respective coils are designated byReference characters a to d. In accordance with magnetic poles that,through electric currents being applied to the coils, appear at therespective top ends of the pillar portions, the direction of a combinedmagnetic field to be applied to the magneto-optical element 120 can bechanged. As illustrated in FIG. 7B, in the case where, through the coilsa and b, magnetic poles S and N are respectively generated, a magneticfield H₁ occurs oriented at −45° from the optical axis that passesthrough the magneto-optical element 120; in contrast, in the case where,through the coils c and d, magnetic poles N and S are respectivelygenerated, a magnetic field H₂ occurs oriented at +45° from the opticalaxis that passes through the magneto-optical element 120. In the casewhere both H₁ and H₂ are applied, a combined magnetic field HC generatedthrough H₁ and H₂ is applied to the magneto-optical element 120.Accordingly, by controlling the values or the directions of the electriccurrents applied to the respective coils, the combined magnetic field HCcan be applied in any direction.

By controlling the values or the directions of the electric currentsapplied to the respective coils, it is possible to make the direction ofthe combined magnetic field turn 360 degrees. FIG. 8 illustrates thetypical directions of the combined magnetic field. In FIG. 8, conditionsin the case where the combined magnetic field is changed by 45°successively are represented in states 1 to 8. It can be seen from FIG.8 that, when magnetic fields are generated by applying electric currentsto the coils, two poles, at the top ends of the pillar portions, thatare generated through the coils (a and b, or c and d) opposing eachother with respect to the magneto-optical element 120 have respectivepolarities that are reverse to each other. Paying attention to the fact,it can be seen that the two coils can be connected to a monopolar powersupply unit in such a way as to generate respective polarities that arereverse to each other. Accordingly, even through there are four coils,two coils can be driven as a pair by a common power supply unit;therefore, only two power supply units are required, and a relativelysimple control system may be utilized.

FIGS. 9A and 9B illustrate examples of coil-connection methods. FIG. 9Aillustrate an example in which the coils a and b are connected inparallel and connected to a first common power supply unit 122, thecoils c and d are connected in parallel and connected to a second commonpower supply unit 123, and control is implemented by a control device124. FIG. 9B illustrate an example in which the coils a and b areconnected in series and connected to a first common power supply unit126, the coils c and d are connected in series and connected to a secondcommon power supply unit 127, and control is implemented by a controldevice 128. In this situation, reference characters a to d correspond tothose in FIG. 7A. Each power supply unit may be a variable voltagesource or a variable current source. By controlling the coil currents bymeans of the control devices 124 and 128 such as a CPU, the magnitudeand the direction of the combined magnetic field can be controlled. Thecombined magnetic field may have constant magnitude to such an extent ascan magnetically saturate the magneto-optical element, regardless of themagnetization direction. In the present invention, because there is nofixed magnetic field generated by a permanent magnet, it is notnecessary to excessively magnify the combined magnetic field, wherebythe number of coil windings can be reduced, and the magneto-opticaldevice can be downsized.

When a plurality of coils that are in parallel with one another isconnected to a power supply unit, the total resistance value is reduced.For example, if two coils having resistance R are connected in parallel,the total resistance R_(p) is R/2; if two coils having resistance R areconnected in series, the total resistance R_(s) is 2R; therefore, if thetotal resistances are compared with each other, R_(p) is a quarter ofR_(s). In the case of parallel connection, the current I that flows ineach coil is V/R. In the case of series connection, the current I thatflows in each coil is V/2R; should the same current I is required inboth the series connection and the parallel connection, the voltage tobe applied in the case of the parallel connection is half of that in thecase of the series connection, whereby low-voltage drive is enabled.Because coils that configure the pair are the same in the number ofwindings and approximately the same in the length of the wire, theirresistances are approximately the same; therefore, in the case of theparallel connection, approximately the same current can flows througheach coil.

Magneto-optical devices illustrated in FIGS. 6A to 6C can be produced,for example, in the following way. By cutting notches, from twodirections that are perpendicular to each other, in arectangular-parallelepiped block made of a high-magnetic-permeabilitymaterial such as NiCuZn-system, a magnetic yoke 114 having fourintegrated pillar portions 112 can be created (refer to FIG. 6A). Itgoes without saying that the magnetic yoke 114 may be formed throughpress molding and baked to a desirable shape. Respective air-core coils116 that have preliminarily been produced are mounted and fixed, throughan adhesive or the like, on the four pillar portions 112 (refer to FIG.6B). In this case, if the air-core coil 116 is made of an enamel-coatedwire material, the air-core coil 116 may be fixed on the pillar portion112 by coating an organic solvent such as ethyl alcohol on the air-corecoil 116 to dissolve the enamel after mounting the air-core coil 116 onthe pillar portion 112. Because, in the foregoing configuration, themagnetic yoke 114 has a unified structure, magnetic resistance thereofcan be suppressed to a minimum, whereby a high-efficiency magneticcircuit can be obtained. Next, the stage 118, made of a non-magneticmaterial, to which the magneto-optical element 120 is adhesively fixed,is fixed through an adhesive or the like on the top ends of the pillarportions 112 (refer to FIG. 6C). As the magneto-optical element 120, forexample, a bismuth-substituted rare-earth iron-garnet single crystal isutilized. The single crystal can be grown through the LPE (liquid-phaseepitaxy) method. Additionally, a YIG (yttrium iron garnet) singlecrystal may be utilized.

Besides, two pairs of the coils that are arranged diagonally opposingeach other, with respect to the magneto-optical element, are connectedin parallel (refer to FIG. 9A) or in series (refer to FIG. 9B) to thepower supply unit. In this situation, the polarities of magnetic fieldsto be applied to the magneto-optical element are made reverse to eachother. Compared with a method in which, with four coils connected torespective power supply units, the four power supply units arecontrolled to adjust the magnitude and direction of the combinedmagnetic field, the control is far facilitated, and costs are reduced.

As illustrated in FIG. 7B, the magnetic field generated through a firstcircuit is designated by H₁; the magnetic field generated through asecond circuit is designated by H₂. H₁ is oriented at −45° from theincident optical axis, and H₂, +45°. The foregoing magnetic-fielddirections are angles obtained through magnetic yokes, as illustrated inFIGS. 6A to 6C, whose structures are inexpensive and readily producible.In this case, the direction θ_(c) of the combined magnetic field isgiven by the following equation:θ_(c)=45−tan⁻¹(H ₁ /H ₂)Because the magnitudes of the magnetic fields generated through therespective circuits are proportional to the values of the currents, H₁and H₂ in the above equation can be replaced by a current I₁ that flowsin the first circuit and a current I₂ that flows in the second circuit,respectively. Because the following equation is given, it can be seenthat, by controlling the currents I₁ and I₂, a desired Faraday rotationangle can be obtained.θ_(f)=θ_(f0)×cos θ_(c)

In the case where a magneto-optical is utilized within the range, of theFaraday rotation angle θ_(f), from 0° to θ_(fMAX), mostly utilized is amagneto-optical element whose Faraday rotation angle θ_(f0) in the casewhere the direction of the saturation combined magnetic field is inparallel with the optical axis is equal to θ_(fMAX). FIG. 10A is a graphrepresenting a relationship between θ_(f) and the current I₁ or I₂, inthe case where a magneto-optical element having θ_(f0) of 90° isutilized. In this case, when the direction θ_(c) of the combinedmagnetic field is requested to be smaller than 45°, it is necessary toreverse the polarity of H₁. In other words, it is necessary to make thecurrent flow reversely. It can be seen from FIG. 10A that the polarityof I₁ reverses at the θ_(f) of approximately 64°. The fact is becausethe maximal Faraday rotation angle θ_(fMAX) in the case of the actualuse is equal to θ_(f0). The above configuration enables θ_(f) to bevaried from 0° to 90°; however, it requires bipolar power supply units,thereby raising the costs. Additionally, there is an inflection point inthe current I₂, whereby control of the currents is rendered slightlycomplicated.

The problem can be solved, by making the Faraday rotation angle θ_(f0)of the magneto-optical element in the case where the direction of thesaturation combined magnetic field is in parallel with the optical axisbe larger than θ_(fMAX) so that θ_(f) becomes equal to θ_(fMAX) orlarger when the magnetic field to be applied consists of H₂ only. Forexample, in the case where, as the foregoing example, H₂is oriented at45° from the incident optical axis, the magnetic field to be appliedconsists of H₂ only, and θ_(f0) is 90°, θ_(f) is given the followingequation:θ_(f)=90×cos 45=63.6°Accordingly, θ_(f) is small by 26.4° in comparison to the requiredθ_(fMAX), 90°.In this case, if a magneto-optical element having a large θ_(f0), e.g.,127.3°, is utilized, θ_(f) is given by the following equation;therefore, without applying a magnetic field reverse to H₁, θ_(fMAX) canbe satisfied.θ_(f)=127.3×cos 45=90°In other words, if the following equation is yielded, the foregoingproblem can be solved.θ_(fMAX)=θ_(f0)×cos θ_(c)

FIG. 10B is a graph representing a relationship between the Faradayrotation angle θ_(f) and the current I₁ or I₂, in the case where amagneto-optical element is utilized that has the Faraday rotation angleθ_(f0) of 127.3° when the direction of the saturation combined magneticfield is in parallel with the optical axis. From FIG. 10B, it can beseen that there is no reversal of polarity in the current I₁ and noinflection point in the current I₂. Accordingly, the foregoingconfiguration makes it possible to utilize monopolar power supply unitsand facilitates the control of the currents.

FIGS. 11A to 11C are views illustrating another example of the magneticyoke. The magnetic yoke is configured in such a way that through-holes132 are provided at the four corners of a square tabular portion 130made of ferrite or the like, cylindrical members 134 made of ferrite orthe like are inserted into the through-holes 132 (refer to FIG. 11A) andfixed thereto through an adhesive or the like (refer to FIG. 11B) Asdescribed above, it is possible to produce relatively readily a magneticyoke that is substantially unified. A magneto-optical element 138 isfixed in such a way as to be situated in the center of the top ends ofthe four cylindrical members 134. For that purpose, for example,respective holes (unillustrated) are provided at the four corners of asquare-tabular non-magnetic stage 140 so that the cylindrical membersare automatically positioned at and fixed to the respective holes, andwith the holes as reference points, a hole is provided at which themagneto-optical element 138 is positioned and fixed; in this way, themagneto-optical element 138 can readily be fixed in the center of thetop ends of the four cylindrical members 134 (refer to FIG. 11C).

As illustrated in FIG. 12, in order to further simplify the control ofthe currents, the first magnetic field H₁ is made to be in a direction,perpendicular to the incident optical axis, in which the Faradayrotation is 0°, and the second magnetic field H₂ is set to be tilted byθ_(c) from the incident optical axis. Accordingly, if θ_(f) is requestedto be zero, only I₁ is supplied; and if θ_(f) is requested to beθ_(fMAX), only I₂ is supplied. FIG. 13 is a graph representing arelationship between the Faraday rotation angle θ_(f) and the current I₁or I₂, in the case where a magneto-optical element is utilized that hasthe Faraday rotation angle θ_(f0) of 127.3° when the direction of thesaturation combined magnetic field is in parallel with the optical axis.The angle θ_(c) is 45°. It can be seen that both I₁ and I₂ can linearlybe varied for θ_(f). Because, in this case, θ_(h)=90−θ_(c),θ_(f)=θ_(fMAX), and θ_(c)=cos⁻¹(θ_(fMAX)/θ_(f0)), θ_(h) can be renderedby the following equation:θ_(h)=90−cos⁻¹(θ_(fMAX)/θ_(f0))=sin⁻¹(θ_(fMAX)/θ_(f0))In other words, if the above equation is satisfied, the control can besimplified.

As illustrated in FIG. 14, a magnetic yoke that can embody the aboveequation can be produced by cutting off three pertinent parts (twoparallel parts and one part perpendicular to the others) of arectangular-parallelepiped block made of a high-magnetic-permeabilitymaterial. In a magnetic yoke 144, compared with the magnetic-polepositions for H₁, the magnetic-pole positions for H₂ are far from thecenter of a magneto-optical element 146, whereby the magnitude of themagnetic field H₂ to be applied to the magneto-optical element 146 isreduced; however, the reduced magnitude can be addressed, e.g., byincreasing the number of windings. In addition, instead of asingle-piece structure, by providing through-holes at predeterminedpositions in the tabular portion and inserting and fixing pillar membersinto the through-holes, the magnetic yoke may be produced, as is thecase with FIGS. 11A to 11C.

FIGS. 15A to 15C are explanatory views further illustrating anotherexample of a magneto-optical device according to the present invention.Two magnetic yokes 150 as illustrated in FIGS. 6A to 6C are arrangedopposing each other. In other words, each of the two magnetic yokes 150has a single-piece structure, made of a high-magnetic-permeabilitymaterial such as ferrite, that incorporates an approximately squaretabular portion 152 and four pillar portions 154 protrudingperpendicularly from four corners in the one surface of the tabularportion 152. Besides, coils 156 are wound around the respective pillarportions 154 (refer to FIG. 15A). The two magnetic yokes 150 aroundwhich the coils 156 are wound as described above are integrated in sucha way that the foremost surfaces of the pillar portions 154 of the onemagnetic yoke 150 butt against the foremost surfaces of the pillarportions 154 of the other magnetic yoke 150, and fixed through anadhesive or the like (refer to FIG. 15B). A cylindrical support 162 towhich a magneto-optical element 160 is adhered is inserted into athrough-hole provided in the center of a non-magnetic stage 158, andfixed therein (refer to FIG. 15B); the stage 158 is fixed to themagnetic yoke 150, through adhesion or the like, in such a way that themagneto-optical element 160 is situated in the open-magnetic-circuitspace surrounded by the top ends of the respective pillar portions 154(refer to FIG. 15C).

Accordingly, the magneto-optical device is configured in such a way thatmagnetic fields generated by the coils 156 are applied to themagneto-optical element 160. For that purpose, currents are applied tothe coils in such a way that the directions of magnetic fields generatedthrough the coils wound around the opposing pillar portions 154 arereverse to each other. That is to say, the currents are applied to thecoils in such a way that, at the top ends of the opposing pillarportions 154, the respective magnetic poles generated through therespective coils 156 have the same polarity. As a result, magneticfluxes leak out of the top ends of the pillar portions 154, whereby themagnetic fields efficiently act on the magneto-optical element 160. Asis the case with FIGS. 6A to 6C, the polarities of magnetic fieldsapplied to the magneto-optical element 160, through the coils 156 thatare diagonally opposing each other with respect to the magneto-opticalelement 160, are reverse to each other. Moreover, the magneto-opticaldevice is configured in such a way that the magnetic fields generatedthrough the upper magnetic yoke and the lower magnetic yokecooperatively (added to each other) act on the magneto-optical element160.

In addition, a set of four coils two each of which are diagonallyopposing each other with respect to the magneto-optical element isdriven by a common power supply unit. When FIG. 16A represents apositional relationship among the coils, as illustrated in FIG. 16B,totally four coils (a₁, a₂, b₁, and b₂), consisting of two coilsvertically opposing each other with respect the incident optical axisand two other coils each diagonally opposing the two coils with respectto the magneto-optical element, that are wired in parallel with oneanother are connected with the one power supply unit; the residual fourcoils (c₁, c₂, d₁, and d₂) are also wired in parallel with one anotherand connected with the other power supply unit. Thus, the coilsvertically opposing each other with respect the incident optical axis,e.g., a₁ and a₂ are set for N poles, and the coils b₁ and b₂ eachdiagonally opposing coils a₁ and a₂ with respect to the magneto-opticalelement are set for S poles. Similarly, if c₁ and c₂ are set for Spoles, d₁ and d₂ are set for N poles.

Additionally, as illustrated in FIG. 16C, the four coils (a₁, a₂, b₁,and b₂) and the residual four coils (c₁, c₂, d₁, and d₂) that are wiredin series may be connected with the one power supply unit and the otherpower supply unit, respectively. Alternatively, a method is alsopossible in which two each of the coils are wired in parallel-series, orin series-parallel, and connected with a common power supply unit. Atany rate, compared with a method in which eight coils that are connectedwith respective power supply units are controlled separately, thecircuit configuration can significantly be simplified, the costs arereduced, and the control is not rendered complicated. A control methodto obtain a desired Faraday rotation angle is the same as that in thecase where the magnetic yoke is provided only at one side.

FIGS. 17A and 17B are explanatory views illustrating another example ofa magneto-optical device according to the present invention. Four pillarportions 166 are arranged between tabular portions 164 that are spacedapart from and opposing each other, and 2 coils 156 are wound aroundeach of the pillar portions 166. The magneto-optical device isconfigured in such away that, by arranging the magneto-optical element160 in a space surrounded by middle portions of the respective pillarportions 166, magnetic fields generated by the coils 156 are applied tothe magneto-optical element 160. By providing holes 165 at the fourcorners of both tabular portions 164, inserting thereinto both ends ofthe respective pillar portions 166, and fixing therein the ends of thepillar portions 166, through an adhesive or the like, a single-piecemagnetic yoke made of a high-magnetic-permeability material can beobtained. Two coils wound around the same pillar portion 166 generatemagnetic fields that are reverse to each other; the directions ofmagnetic fields applied, through the coils diagonally opposing eachother with respect to the magneto-optical element 160, to themagneto-optical element 160 are reverse to each other; assuming that themiddle portion of the pillar portion is a boundary, the magnetic fieldgenerated through the top-half magnetic yoke and the magnetic fieldgenerated through the bottom-half magnetic yoke cooperatively act on themagneto-optical element; and the coils opposing each other with respectto the magneto-optical element 160 is driven, as a set, by a commonpower supply unit. The operation of the magneto-optical device is thesame as that of the foregoing embodiments.

In addition, the foregoing embodiments are examples in which four pillarportions are provided in a tabular portion; however, a structure mayalso be possible in which more pillar portions, e.g., six pillarportions are provided.

FIG. 18 is a diagram illustrating an application example of areflection-type variable optical attenuator including a magneto-opticaldevice according to the present invention. A configuration is employedin which, before the magneto-optical device 170, a light polarizer 172made of a birefringence crystal and a lens 174 are arranged along thelight path, and after the magneto-optical device 170, a mirror 176 isarranged along the light path. The magneto-optical device 170 may beidentical to that illustrated in FIGS. 6A to 6C; for simplicity,corresponding members are designated by the same reference characters.Light is made to enter through an input optical fiber, and output lightis extracted through an output optical fiber. The example employs aconfiguration in which coils are connected in series (refer to FIG. 9B).

FIGS. 19A and 19B are graphs representing an example of results ofmeasurement on a prototype. FIG. 19A represents the optical-attenuationproperties versus the current of Coil 1 (the coils a and b) in the casewhere the current of Coil 2 (the coils c and d) is fixed to 80 mA.Additionally, FIG. 19B represents the response properties versus thestep input to Coil 1 with the attenuation value of 16 dB or 26 dB. Inthe case where the attenuation value was 26 dB, the response timeconstant (attenuation value: reaching down to 63%) was 24 μsec.

EMBODIMENT 3

Embodiment 3 of a magneto-optical device according to the presentinvention will be explained with reference to FIGS. 20 to 28. Asillustrated in FIG. 20, a magneto-optical device according to Embodiment3 includes a magnetic yoke 210, coils 215 wound around the magnetic yoke210, and a magneto-optical element 220. The magnetic yoke 210 has aconfiguration in which a disk-shaped tabular portion 211 and fourinverted L-shaped pillar portions 212 protruding from a peripheralportion of the one surface of the tabular portion 211 are incorporated,the respective pillar portions 212 are arranged, along the circumferenceof the tabular portion 211, spaced approximately the same distance apartfrom one another, and the respective top ends of the pillar portions 212are facing the center of the pillar portions 212. The tabular portion211 is made of, e.g., a semi-hard magnetic material such as SUS420J2;the pillar portions 212 are made of a soft-magnetic material such asMn—Zn-system ferrite. The tabular portion 211 and each of the pillarportions 212 are fixed to each other, through an adhesive or the like.

The coils 215 are wound around the respective pillar portions 212, andthe magneto-optical element 220 is arranged in an open-magnetic-circuitspace surrounded by the top ends of the four pillar portions 212;accordingly, the combined magnetic field generated through the coils 215is applied to the magneto-optical element 220. As is the case withEmbodiments 1 and 2 described above, the polarities of magnetic fieldsapplied to the magneto-optical element 220, through the coils 215 thatare diagonally opposing each other with respect to the magneto-opticalelement 220, are reverse to each other; the coils 215 are connected inparallel or in series with a power supply unit. In addition, the numberof windings of the coil 215 can arbitrarily be set, and, by changing thenumber of windings, the magnetomotive force of the coil 215 can beadjusted; however, for simplicity, explanation will be implemented onthe assumption that the magnetomotive force is adjusted through thecurrent supplied to the coil 215.

In the magneto-optical device configured as described above, bysupplying currents to the four coils 215, the magnetomotive force isgenerated in each of the coils 215. Besides, by controlling thedirections and the ratios of currents supplied to the four coils 215, acombined magnetic field (spatial magnetic field) having an arbitrarydirection can be formed in an open-magnetic-circuit space as anobjective space. When respective currents are supplied to the coils 215,for example as illustrated in FIG. 21A, the respective pillar portions212 of the magnetic yoke 210 are magnetized in a predetermined direction(e.g., in the direction indicated in FIG. 21A); however, the tabularportion 211 made of a semi-hard magnetic material is also magnetized ina predetermined direction (e.g., in the direction indicated in FIG.21A), while forming a magnetic circuit along with the pillar portions212. Because the magnetization direction of the tabular portion 211 hasa distribution, in effect, the magnetization is more complicated thanthat illustrated in FIG. 21A; however, the magnetization as a whole ofthe tabular portion 211 is exemplified, as illustrated in FIG. 21A.

In this situation, if exciting currents for the respective coils 215 arecut off, the magnetomotive force generated through the coils 215disappear; however, magnetization remains in the tabular portion 211that is made of a semi-hard magnetic material. After the excitingcurrents have been cut off, as illustrated in FIG. 21B, due to theresidual magnetization in the tabular portion 211, the pillar portions212 that are made of a soft-magnetic material are magnetized, whereby acombined magnetic field having the same direction as that of themagnetization in the case where the currents are applied is formed inthe open-magnetic-circuit space as an objective space. In other words,by controlling the ratios of the exciting currents applied to therespective coils 215, the residual magnetization direction as a whole ofthe tabular portion 211 can arbitrarily be changed; as a result, thedirection of a magnetic field that remains in the open-magnetic-circuitspace can arbitrarily be changed. In magnetizing the tabular portion 211in a target direction, the absolute values of the exciting currentsapplied to the respective coils 215 are increased, with the ratios ofthe exciting currents maintained. It is preferable in terms of controlto adjust the values of the currents so that the pillar portions 212 arenot magnetically saturated.

In addition, in changing the direction of the combined magnetic fieldapplied to the magneto-optical element 220, it is preferable to applyelectric power that is large enough, compared with the residualmagnetization in the tabular portion 211, or to demagnetize the tabularportion 211, by controlling the currents supplied to the respectivecoils 215, and then to form a combined magnetic field in theopen-magnetic-circuit space. Accordingly, the effect of the residualmagnetization before the change of the direction of the combinedmagnetic field is nullified, whereby the direction of the combinedmagnetic field can appropriately be controlled in a desired direction.Moreover, in the foregoing magnetic circuit in FIG. 20, the direction ofthe combined magnetic field applied to the magneto-optical element 220can be controlled so as to be oriented arbitrarily on a plane (a planeon which the top ends of the respective pillar portions 212 aresituated); by combining with that magnetic circuit that can generate amagnetic-field vector having a component in a direction perpendicular tothe plane, as illustrated in FIGS. 22 and 23, the direction of thecombined magnetic field applied to the magneto-optical element 220 canbe controlled so as to be oriented arbitrarily in a 3-dimensional space.

In FIG. 22, the magnetic yoke 230 is comprised of a block-shaped baseportion 231 made of a semi-hard magnetic material and six pillarportions 232, 233, and 234 protruding from the top and side surfaces ofthe base portion 231. The base portion 231 has, e.g., a cuboid shape;the four pillar portions 232 protrude perpendicularly from thevicinities of the four corners of the top surface of the base portion231, and the respective top ends of the pillar portions 232 are orientedto a open-magnetic-circuit space that is surrounded by the top ends;from the one side surface of the base portion 231, the pillar portions234 and 233 protrude that extend from the upper and the lower portions,respectively, of the side surface, and whose top ends lead to a positionbelow and a position above the open-magnetic-circuit space,respectively. Coils 235 are wound around the respective pillar portions232, 233, and 234. In contrast, in FIG. 23, a first magnetic yoke 210having the four pillar portions 212 as illustrated in FIG. 20 describedabove and a second magnetic yoke 240 having two pillar portions 242 areprovided; arrangement is implemented in such a way that anopen-magnetic-circuit space surrounded by the top ends of the pillarportions 212 of the first magnetic yoke 210 are sandwiched between thetop ends of the two pillar portions 242 of the second magnetic yoke 240,from the upper and lower sides thereof. A tabular portion 241 of thesecond magnetic yoke 240 is formed of a semi-hard magnetic material, andthe pillar portion 242 is formed of a soft-magnetic material. Coils 245are wound around the respective pillar portions 242. In both cases, thedirection of the combined magnetic field applied to the magneto-opticalelement 220 can be controlled so as to be in an arbitrary direction in a3-dimentional space, and the direction can be maintained after theexciting currents have been cut off.

As an example of a magneto-optical device according to the presentinvention, a Faraday rotator as illustrated in FIG. 20 was produced. Forthe pillar portion 212, silicon steel that is a soft-magnetic materialand 3 mm by 3.3 mm in end face and 14 mm in height was utilized. For thetabular portion 211, SUS420J2 that is a semi-hard magnetic material and40 mm in diameter and 0.4 mm in thickness was utilized. For themagneto-optical element 220, bismuth-substituted garnet single crystalof 1 mm by 1.2 mm by 0.98 mm in size was selected, and a crystal lengthwas selected with which, in the case of input light having a wavelengthof 1550 nm, the maximal value of the Faraday rotation angle becomes 90°.In addition, the number of windings of each of the coils 215 was set to800; the coils 215 that are diagonally opposing each other with respectto the magneto-optical element 220 were connected in series with a powersupply unit, and two power-supply systems were utilized. As illustratedin FIG. 24, if, out of two pairs of coils that are connected in series,the one pair is designated by 215 a and the other is designated by 215b, and the currents to be applied to the respective pairs of coils aredesignated by current 1 and current 2, respectively, the angle θ betweenthe direction of the combined magnetic field in theopen-magnetic-circuit space and the traveling direction of a ray, andthe current values of the respective current systems had a relationshiptherebetween as represented in FIG. 25A. It can be seen that bycontrolling the currents 1 and 2, a combined magnetic field can beformed in an arbitrary direction in the open-magnetic-circuit space.

With a self-latching Faraday rotator configured as described above, bysetting the maximal values of the currents applied to the respectivecoils 215 a and 215 b to 250 mA and controlling the ratios and thedirections of the currents, the Faraday rotation angle can arbitrarilybe adjusted over a range from −90° to +90°, and after the currents forthe respective coils 215 a and 215 b are cut off, the Faraday rotationangle in the case where the currents have been applied can bemaintained. In other words, a self-latching Faraday rotator can berealized in which the Faraday rotation angle after the currents for therespective coils 215 a and 215 b are cut off is an arbitrary angle overa range from −90° to +90°. FIG. 25B represents the results ofmeasurement on the Faraday rotation angle in the case where the angle θbetween the direction of the combined magnetic field in theopen-magnetic-circuit space and the traveling direction of a ray is overa range from −45° to +135°. It can be seen that, even after the currentsfor the coils 215 a and 215 b are cut off, the Faraday rotation anglecan be maintained in the condition that is approximately the same asthat in the case where the currents have been applied.

FIG. 26 is an explanatory view illustrating an example of aself-latching variable optical attenuator including a magneto-opticaldevice according to the present invention. A configuration is employedin which, before and after the magneto-optical device, a light polarizer221 and an analyzer 222 are arranged along the light path. Themagneto-optical device may be identical to that illustrated in FIG. 20;for simplicity, corresponding members are designated by the samereference characters.

In this situation, for the four pillar portions 212, silicon steel thatis a soft-magnetic material was utilized, and for the tabular portion211, SUS410J1 that is a semi-hard magnetic material was utilized; heattreatment was applied to both materials, under appropriate conditions.The number of windings of each of the coils 215 was set to 400; aconfiguration was employed in which the coils 215 that are diagonallyopposing each other and wired in series are connected with a powersupply unit in such a way that the polarities that appear at therespective top ends of the pillar portions 212 opposing each other withrespect to the magneto-optical element 220 are reverse to each other,and the control is implemented through two current systems. Themagneto-optical element 220 is a bismuth-substituted garnet singlecrystal; the light polarizer 221 and the analyzer 222 are rutile singlecrystals. The magneto-optical element 220 was 1 mm by 1.7 mm by 0.98 mmin size, and a crystal length was selected with which, in the case ofinput light having a wavelength of 1550 nm, the maximal value of theFaraday rotation angle becomes 90°. The rutile single crystalsconfigured a cross-Nicol arrangement, so that, in the case where therotation angle of the Faraday rotator was 90°, the minimal attenuationvalue was obtained, and in the case where the rotation angle of theFaraday rotator was 0°, the maximal attenuation value was obtained.

According to the self-latching variable optical attenuator configured asdescribed above, by setting the maximal values of the currents appliedto the respective coils 215 to 250 mA and controlling the ratios and thedirections of the currents, the Faraday rotation angle can be convertedinto a desired angle, and, as represented in FIG. 27, the value of lightattenuation can be adjusted to an arbitrary value over a range from 1 dBto 22 dB. Besides, after the currents for the respective coils 215 arecut off, the Faraday rotation angle is maintained in the condition asthe currents have been applied; therefore, the value of lightattenuation can also be maintained in the condition as the currents havebeen applied. In other words, a self-latching variable opticalattenuator can be realized in which, the value of light attenuationobtained after the currents for the respective coils 215 are cut off isarbitrary over a range from 1 dB to 22 dB. The device is 47 mm by 47 mmby 25 mm in size.

FIG. 28 is an explanatory view illustrating an example of aself-latching variable optical splitter including a magneto-opticaldevice according to the present invention. The optical splitter isconfigures in such a way that, from the input-port side to theoutput-port side, a rutile single crystal plate 251 a, a magneto-opticaldevice, ½-wavelength plates 252 a and 252 b, a rutile single crystalplate 251 b, ½-wavelength plates 252 c and 252 d, and a rutile singlecrystal plate 251 c are arranged in that order. The magneto-opticaldevice is a self-latching Faraday rotator, among self-latching Faradayrotators as illustrated in FIG. 20, that utilizes the magneto-opticalelement 220 that provides a Faraday rotation angle of 90°. In FIG. 28,the magnetic-circuit portion of the Faraday rotator is omitted. Inaddition, the directions of the crystal axes of the three rutile singlecrystal plates 251 a, 251 b, and 251 c are indicated by the arrows inFIG. 28; the rutile single crystal plate 251 a has a function ofsplitting a ray consisting of subrays that are in the same light pathand whose polarization directions are perpendicular to each other; therutile single crystal plate 251 b has a function of controlling a lightpath in accordance with a polarization direction; and the rutile singlecrystal plate 251 c has a function of synthesizing rays that are indifferent light paths and whose polarization directions areperpendicular to each other. The four ½-wavelength plates 252 a, 252 b,252 c, and 252 d each have a function of rotating by a predeterminedangle the polarization direction of a ray.

In the self-latching variable optical splitter configured as describedabove, an incident ray to the input port are separated into two subrays,and the subrays are emitted through output port 1 and output port 2; theseparation ratio can be controlled through the values of currentssupplied to the Faraday rotator. The separation ratio excluding theinsertion loss can be varied over a range from 0:100 to 100:0; obtainedcrosstalk values were above 42 dB. After the currents for the Faradayrotator are cut off, the Faraday rotation angle is maintained in thecondition in the case where the currents have been applied; therefore,as is the case where the currents have been applied, an arbitraryseparation ratio can be maintained.

By combining the magneto-optical device illustrated in FIG. 20, a rutilesingle crystal plate, and a ½-wavelength plate, it is possible toconfigure a self-latching variable optical switch. According to theself-latching variable optical switch, by controlling the ratios anddirections of the currents applied to the respective coils 215, thelight path can be switched so as to be in a desired condition, and afterthe currents for the coils 215 are cut off, the condition can bemaintained. Moreover, in Embodiment 3, the pillar portion is formed of asoft-magnetic material, and the tabular portion is formed of a semi-hardmagnetic material; conversely, it is possible that the pillar portion isformed of a semi-hard magnetic material, and the tabular portion isformed of a soft-magnetic material. In this case as well, after theexciting currents are cut off, the direction of the combined magneticfield that remains in the open-magnetic-circuit space can be controlledso as to be in an arbitrary direction, whereby the Faraday rotationangle can be maintained so as to be in an arbitrary condition.

INDUSTRIAL APPLICABILITY

A magneto-optical device according to the present invention can beutilized in a variable optical attenuator, a variable optical splitter,or the Faraday rotator in a optical switch. By generating variablemagnetic fields by utilizing electromagnets only, without utilizing anyfixed magnetic field of a permanent magnet, the magnetization directioncan instantaneously be changed, whereby the magneto-optical device canbe downsized and its operation is speeded up. Since, regardless of thedirection of the combined magnetic field, no large magnetic field isrequired, the coils can be downsized, whereby the driving voltage canalso be reduced.

Since a pair of coils diagonally opposing each other with respect to amagneto-optical element is configured and the coils in the pair arewired in parallel or in series and driven by a common power supply unit,the drive of the coils can efficiently be implemented with a smallnumber of power supply units, whereby the peripheral circuitry can besimplified.

Moreover, by forming with a semi-hard magnetic material the tabularportion or the pillar portion of the magnetic yoke, even after theexciting currents are cut off, the direction of the combined magneticfield that remains in the open-magnetic-circuit space can be controlledso as to be in an arbitrary direction, whereby the Faraday rotationangle can be maintained so as to be in an arbitrary condition.

1. A magneto-optical device comprising: a magnetic yoke made of ahigh-magnetic-permeability material, the magnetic yoke including atabular portion, and at least three pillar portions protrudingperpendicularly from one side of the tabular portion; coils wound aroundthe pillar portions; and a magneto-optical element arranged in anopen-magnetic-circuit space surrounded by the respective top-endportions of the pillar portions, wherein magnetic fields generatedthrough the coils are applied to the magneto-optical element.
 2. Themagneto-optical device according to claim 1, wherein the magnetic yokehas a structure in which an approximately quadrangular tabular portionand four quadrangular pillar portions protruding from the vicinities ofthe four corners of the tabular portion, perpendicularly and in the samedirection, are continuously integrated.
 3. The magneto-optical deviceaccording to claim 1, wherein as the high-magnetic-permeabilitymaterial, ferrite is utilized, and for the magneto-optical element, abismuth-substituted rare-earth iron-garnet single crystal is utilized.4. The magneto-optical device according to claim 1, wherein, bycontrolling respective directions and/or values of electric currentssupplied to the coils, the magnetization direction of themagneto-optical element can be changed.
 5. The magneto-optical deviceaccording to claim 1, wherein the magnetic yoke has a structure having2n (where n≧2) pillar portions protruding perpendicularly from one sideof the tabular portion; the polarities of magnetic fields applied to themagneto-optical element, through the coils diagonally opposing eachother with respect to the magneto-optical element, are reverse to eachother; and a pair of the coils that are diagonally opposing each otherwith respect to the magneto-optical element and connected in parallel orin series is driven by a common power supply unit.
 6. Themagneto-optical device according to claim 1, wherein the tabular portionof the magnetic yoke is formed of a semi-hard magnetic material, and thepillar portion is formed of a soft-magnetic material.
 7. A variableoptical attenuator for, with light polarizers arranged before and aftera magneto-optical device in the light path thereof, controlling anattenuation value of output optical power versus input optical power,the magneto-optical device comprising: a magnetic yoke made of ahigh-magnetic-permeability material, the magnetic yoke including atabular portion, and at least three pillar portions protrudingperpendicularly from one side of the tabular portion; coils wound aroundthe pillar portions; and a magneto-optical element arranged in anopen-magnetic-circuit space surrounded by the respective top-endportions of the pillar portions, wherein magnetic fields generatedthrough the coils are applied to the magneto-optical element, and themagnetization direction of the magneto-optical element can be changed,by controlling respective directions and/or values of electric currentssupplied to the coils.
 8. An optical switch for controlling switchedlyoutput light versus input light, the optical switch having aconfiguration in which light polarizers are arranged before and after amagneto-optical device in the light path thereof, andresidual-magnetization-natured bismuth-substituted rare-earth irongarnet single crystal is utilized as a magneto-optical element, themagneto-optical device comprising: a magnetic yoke made of ahigh-magnetic-permeability material, the magnetic yoke including atabular portion, and at least three pillar portions protrudingperpendicularly from one side of the tabular portion; coils wound aroundthe pillar portions; and a magneto-optical element arranged in anopen-magnetic-circuit space surrounded by the respective top-endportions of the pillar portions, wherein a magnetic field generatedthrough the coils is applied to the magneto-optical element, and themagnetization direction of the magneto-optical element can be changed,by controlling respective directions and/or values of electric currentssupplied to the coils.
 9. A magneto-optical device array in whichmagneto-optical devices are arranged side by side, the magneto-opticaldevice comprising: a magnetic yoke made of a high-magnetic-permeabilitymaterial, the magnetic yoke including a tabular portion and at leastthree pillar portions protruding perpendicularly from one side of thetabular portion; coils wound around the pillar portions; and amagneto-optical element arranged in an open-magnetic-circuit spacesurrounded by the respective top-end portions of the pillar portions,wherein magnetic fields generated through the coils are applied to themagneto-optical element.
 10. A magneto-optical device comprising: amagnetic yoke made of a high-magnetic-permeability material, themagnetic yoke including 2n (where n≧2) pillar portions protrudingperpendicularly from one side of a tabular portion; coils wound aroundthe pillar portions; and a magneto-optical element arranged in anopen-magnetic-circuit space surrounded by the respective top-endportions of the pillar portions, wherein: magnetic fields generatedthrough the coils are applied to the magneto-optical element; thepolarities of magnetic fields applied to the magneto-optical element,through the coils diagonally opposing each other with respect to themagneto-optical element, are reverse to each other; and a pair of thecoils that are diagonally opposing each other with respect to themagneto-optical element and connected in parallel or in series is drivenby a common power supply unit.
 11. The magneto-optical device accordingto claim 10, wherein the magnetic yoke has a structure having anapproximately square tabular portion and four pillar portions protrudingfrom the vicinities of the four corners of the tabular portion,perpendicularly and in the same direction.
 12. The magneto-opticaldevice according to claim 10, wherein the magnetic yoke has a tabularportion and pillar portions protruding in the same direction from oneside of the tabular portion, and one pair of the pillar portionsopposing each other with respect to the magneto-optical element isarranged perpendicular to the optical axis of the magneto-opticalelement and the other pair of the pillar portions opposing each otherwith respect to the magneto-optical element is arranged at a specificangle (smaller than ±90°) from the optical axis.
 13. The magneto-opticaldevice according to claim 12, wherein the angle θ_(h) between thedirection of a first magnetic field formed through the one pair ofpillar portions and the direction of a second magnetic field formedthrough the other pair of pillar portions is given by the followingequation:θ_(h)=sin⁻¹(θ_(fMAX)/θ_(f0)) where θ_(f0) is the Faraday rotation angleof the magneto-optical element in the case where the direction of asaturation combined magnetic field is parallel to the optical axis, andθ_(fMAX) is a maximal Faraday rotation angle of the magneto-opticalelement in the case of actual use.
 14. The magneto-optical deviceaccording to claim 10, wherein, in the case where the direction of asaturation combined magnetic field is parallel to the optical axis ofthe magneto-optical element, the Faraday rotation angle of themagneto-optical element is set to 127.3° or larger, and the coils aredriven by monopolar power supply units.
 15. A magneto-optical devicecomprising: a magnetic yoke having a tabular portion made of a semi-hardmagnetic material and at least three pillar portions, protruding fromone side of the tabular portion, that are made of ahigh-magnetic-permeability material; coils wound around the pillarportions; and a magneto-optical element arranged in anopen-magnetic-circuit space surrounded by the respective top-endportions of the pillar portions, wherein magnetic fields generatedthrough the coils are applied to the magneto-optical element.
 16. Amagneto-optical device comprising: a magnetic yoke having a tabularportion made of a high-magnetic-permeability material and at least threepillar portions, protruding from one side of the tabular portion, thatare made of a semi-hard magnetic material; coils wound around the pillarportions; and a magneto-optical element arranged in anopen-magnetic-circuit space surrounded by the respective top-endportions of the pillar portions, wherein magnetic fields generatedthrough the coils are applied to the magneto-optical element.
 17. Aself-latching variable optical attenuator for, with a light polarizer, amagneto-optical device, and an analyzer arranged in that order,controlling an attenuation value of output optical power versus inputoptical power, the magneto-optical device comprising: a magnetic yokehaving a tabular portion made of a semi-hard magnetic material and atleast three pillar portions, protruding from one side of the tabularportion, that are made of a high-magnetic-permeability material; coilswound around the pillar portions; and a magneto-optical element arrangedin an open-magnetic-circuit space surrounded by the respective top-endportions of the pillar portions, wherein magnetic fields generatedthrough the coils are applied to the magneto-optical element.
 18. Aself-latching variable optical splitter, comprised of a magneto-opticaldevice, a light polarizer, an analyzer, and a waveplate, for controllingoptical separation ratio through the magneto-optical device, themagneto-optical device comprising: a magnetic yoke having a tabularportion made of a semi-hard magnetic material and at least three pillarportions, protruding from one side of the tabular portion, that are madeof a high-magnetic-permeability material; coils wound around the pillarportions; and a magneto-optical element arranged in anopen-magnetic-circuit space surrounded by the respective top-endportions of the pillar portions, wherein magnetic fields generatedthrough the coils are applied to the magneto-optical element.
 19. Amagneto-optical device comprising: a magnetic yoke having a block-shapedbase portion made of a semi-hard magnetic material and at least fourpillar portions, made of a high-magnetic-permeability material,extending from the base portion to the vicinity of a space to be anopen-magnetic-circuit space; coils wound around the pillar portions; anda magneto-optical element arranged in the open-magnetic-circuit spacesurrounded by the respective top-end portions of the pillar portions,wherein magnetic fields generated through the coils are applied to themagneto-optical element, in three or more directions.